Hematopoiesis is maintained by a hierarchical system where hematopoietic stem cells (HSCs) give rise to multipotent progenitors, which in turn differentiate into all types of mature blood cells. The molecular mechanisms controlling multipotentiality, self-renewal, quiescence and HSC commitment have been extensively studied. However, numerous issues remain to be addressed and important genes regulating these processes remain to be identified.
Myeloid malignancies include Acute Myeloid leukaemia (AML), Myeloproliferative disorders (MPDs), myelodysplastic syndromes (MDS) and myelodysplastic/myeloproliferative syndromes that are all clonal stem-cell (HSC) or progenitor malignant disorders (TIU et al., Leukemia, vol. 21(8), p:1648-57, 2007).
Several genetic mutations have been correlated to AML, and four groups are recognized: (i) the AML with recurrent genetic abnormalities AML t(8;21)(q22;q22) with RUNX1-ETO fusion gene; AML with abnormal bone marrow eosinophils and inv(16)(p13;q22) or t(16;16)(p13;q22) with CBFB/MYH11 rearrangement; acute promyelocytic leukaemia APL with t(15;17)(q22;q12) PML/RARA; AML with 11q23 (MLL) abnormalities); (ii) AML with multilineage dysplasia following MDS or MDS/MPD or without antecedent of MDS or MPD; (iii) AML or MDS therapy related and (iv) other unclassified AML among that comprises the group of AML with normal karyotype which prognosis is based on molecular analysis of oncogenes such as mutations of FLT3-ITD or NPM1.
Myelodysplastic/myeloproliferative syndromes include four myeloid diseases grouped in 1999 by the WHO: chronic myelomonocytic leukemia (CMML), juvenile myelomonocytic leukemia (JMML), atypical chronic myeloid leukemia (aCML) and unclassified myelodysplastic/myeloproliferative syndromes (U-MDS/MPS).
MDS include refractory anemia (RA), and refractory cytopenia with multilineage dysplasia (RCMD). MDS are characterized by ineffective hematopoiesis in one or more of the lineage of the bone marrow. Early MDS mostly demonstrate excessive apoptosis and hematopoietic cell dysplasia (CLAESSENS et al., Blood, vol. 99, p:1594-601, 2002; CLASESSENS et al., Blood, vol. 105, p:4035-42, 2005). In about a third of MDS patients, this ineffective hematopoiesis precedes progression to secondary AML (sAML). Although some molecular events associated with specific MDS subtypes (ELBERT et al., Nature, vol. 451(7176), p:335-9, 2008) or disease transformation (BRAUN et al., Blood, vol. 107(3), p:1156-65, 2006) have been identified, the underlying molecular defects are still poorly understood. No biological markers, except morphological features, are currently available for early diagnosis and prognosis.
MPDs, referred now as myeloproliferative neoplasms (MPN; TEFFERI & VARDIMAN, Leukemia, vol. 22, p:14-22, 2008), are chronic myeloid diseases including chronic myelogenous leukaemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF) and idiopathic myelofibrosis (IMF). MPDs are characterized by an increased proliferation of one or several myeloid lineages. If most MPDs are sporadic diseases, familial cases of MPDs, for which the exact prevalence is unknown, have been reported (GILBERT, Baillieres Clin. Haematol., vol. 11, p:849-858, 1998; KRALOVICS et al., Blood, vol. 102, p:3793-3796, 2003; BELLANNE-CHANTELOT et al., Blood, vol. 108, p:346-352, 2006). The clinical analysis of these familial cases has shown that they are phenotypically identical to sporadic cases. Nevertheless, MPD families are characterized by a clinical and genetic heterogeneity. First, MPD cases from a single family can either display the same subtype or different types of MPD (GILBERT, abovementioned, 1998; BELLANNE-CHANTELOT et al., abovementioned, 2006; RUMI et al., Cancer, vol. 107, p:2206-2211, 2006). Second, about 6-15% of patients with PV and 3-5% of patients with ET are at risk of developing hematological complication after 15 years of evolution (FINAZZI & HARRISON, Semin. Hematol., vol. 42, p:230-238, 2005; KILADJIAN et al., Blood, vol. 112, p:1746, 2008; PASSAMONTI et al., Blood, vol. 111, p:3383-3387, 2008; PASSAMONTI et al., Haematologica, vol. 93, p:1645-1651, 2008).
MPDs, in both sporadic and familial cases, are commonly associated with an acquired constitutive kinase activity, as exemplified by the JAK2V617F mutation in Polycythemia Vera, in most PV cases and in half of ET and PMF cases (MORGAN & GILLIGAND, Annu. Rev. Med., vol. 59, p:213-22, 2008; DELHOMMEAU et al., Cell Mol. Life. Sci., vol. 63(24), p:2939-53, 2006, CAMPBELL & GREEN, N. Engl. J. Med., vol. 355(23), p:2452-66, 2006; BELLANNE-CHANTELOT et al., abovementioned, 2006; JAMES et al., Nature, vol. 434, p:1144-1148, 2005; BAXTER et al., Lancet, vol. 365, p:1054-1061, 2005; LEVINE et al., Blood, vol. 106, p:3377-3379, 2005; KRALOVICS et al., N. Engl. J. Med., vol. 352, p:1779-1790, 2005). MPDs frequently result from the expression of a constitutive tyrosine kinase protein:                Through a fusion like BCR-ABL in CML, FIP1L1-PDGFRA in HES, TEL-PDGFRB in CMML with hypereosinophilia, ZNF198-FGFR1 in rare MPD coupled to lymphoid proliferation and PCM1-JAK2 in rare MPDs, AML and T cell lymphomas        A limited or single nucleotide mutation i.e. JAK2 V617F (1849G>T), which recent discovery of in PV (98%), ET (75%), IMF (50%) and a few percent of CMML, MDS/MPD and U-MPD allows for a new MPD classification and diagnosis criteria and perspectives for treatment. In addition, KIT mutations are recurrent in systemic mast cell proliferation.        Through activating mutations in the receptor for thrombopoietin receptor (MPL), especially of the tryptophan 515 (MPLW515K/L/A) (PIKMAN et al., PLoS Med, vol. 3(e270), 2006; CHALIGNÉ et al., Leukemia, vol. 22, p1557-66, 2008).        Marginal cases of CML presented with BCR/JAK2 rearrangement due to t(9;22)(p24;q11).        
The JAK2 gene on chromosome 9p encodes a tyrosine kinase that associates with type 1 cytokine receptors. The V617F mutation is predicted to disrupt the auto-inhibitory effect of the JH2 domain to constitutive activation of the kinase. Wild type JAK2 exerts a dominant negative effect on the activity of the mutated protein. Therefore the loss of WT JAK2 associated to the duplication of the mutated gene by mitotic recombination observed in most of MPD samples allows for a higher expression and activity of the mutated kinase.
However, several observations, such as the Polycythemia Vera co expressing the WT and mutated JAK2 and the characterization of secondary AML emerging from mutated MPD but lacking of JAK2 mutation in the blast phases indicate oncogenetic events earlier occurring before JAK2 mutation. Moreover, and as discussed previously, the MPD disease evolution is indeed highly variable within and between families. Thus, there is some evidence that there is at least one other mutation than JAK2 implicated in MPDs and, more specifically, their progression.
Lymphoid tumours consist of expansion of cells with lymphoid features. Acute lymphoblastic leukaemia/lymphoma are proliferation of cells blocked in lymphoid differentiation, from either T (T-cell acute lymphoblastic leukaemia; T-ALL) or B (B-cell precursor acute lymphoblastic leukaemia; BCP-ALL) origin. Some leukaemia lymphoma are from Natural Killer (NK) origin. Lymphoma involve expansion of more mature lymphoid cells (B or T). Some neoplasms are chronic, and can involve T cell (prolymphocytic leukaemia) or B cells (Chronic Lymphocytic Leukaemia). The classification of lymphoid neoplasm is based on anatomopathological analyses, differentiation markers and pathogenesis data (Swerdllow S. H., Campo E., Harris N. L., Jaffe E. S., Pileri S. A., Stein H., Thiele J. W., Vardiman J. W. (Eds): WHO classification of tumors of haematopoietc and lymphoid tissues. IARC: Lyon 2008). For example, Anaplasic large T-cell lymphoma are associated with NPM-ALK fusion oncogene (and variant thereof), follicular lymphoma are associated with BCL2 activation following t(14;18)(q32;q21) chromosomal translocation, mantle cell lymphoma are associated with CCND1 activation following t(11;14)(q13;q32) chromosomal translocation. Many lymphoma however lack any reliable molecular marker suggesting a pathophysiological mechanism. This is the case, In particular, for more than 50% of diffuse large B cell lymphomas (DLBCL), for most peripheral T-cell lymphomas (PTCL) and for a majority of non-follicular low grade lymphomas.
Therefore, there was an urgent need of a reliable diagnostic marker that allows to identify lymphoid and myeloid neoplasms, in particular MDS and MPD, and eventually to prognosticate their progression.
The Ten Eleven Translocation protein family contains three recently identified members, with unknown functions, characterized in that they share two highly conserved domains at their C-terminal end. As used herein, the expression “gene of the TET family” refers to members of the Ten Eleven Translocation family, TET1, TET2 or TET3, which have been recently identified (Lorsbach et al, Leukemia 2003).
Among them, TET1 is the only studied member, because it has been identified as a fusion partner with the protein mixed lineage leukemia (MLL) in two different and independent studies (ONO et al., Cancer Research, vol. 62(14), p:4075-80, 2002 and LORSBACH et al., Leukemia, vol. 17(3), p:637-41, 2003). This protein, also called LCX, or “leukemia associated protein with a CXXC domain in N-terminal region”, contains an α-helical coiled-coil region in its C-terminal region, region which is retained in the fusion MLL-TET1. On the contrary, the N-terminus CXXC domain of TET1 is not present in this protein fusion (Ono R, Cancer Research 2002). The two highly conserved carboxy terminal regions are included in the MLL-TET1 fusion (Lorsbach et al, Leukemia 2003). One conserved region is disrupted by the translocation; the other one is fused to MLL. Despite its description as an MLL fusion partner 7 years ago, functional and sequence analysis of the TET1 gene have been reported recently, after the priority date of the present application.
The MLL gene is located at human chromosome 11q23 and is found to be rearranged in a heterogenous group of lymphoid, myeloid and mixed lineage human leukemias. More than 70 loci have been described to be rearranged with the 11q23 chromosomal band and at least 50 of these have been cloned and characterized on a molecular level. Most of the MLL rearrangements map to a 8.3 kb base of the genes. The partners genes are always fused in frame to the 5′ part MLL and may include MLL itself. Amplifications of MLL have also been reported. The partner genes code for proteins with disparate functions. In the MLL fusion, they may provide transcriptional activation domains, chromatin modifier complex recruitment or dimerization/oligomerization motif. Indeed, the expression of an MLL-Beta-galactosidase (a bacterial protein able to tetramerize) or to dimerization domain is sufficient to induce leukemia in mouse models. Therefore, it is not possible to infer the function of a protein or its independent involvement in cellular transformation from its fusion to MLL (The versatile mixed lineage leukaemia gene MLL and its many associations in leukaemogenesis. Daser A, Rabbitts T H. Semin Cancer Biol. 2005 June; 15(3):175-88. Review. Chromosomal rearrangements leading to MLL gene fusions: clinical and biological aspects. Harper D P, Aplan P D. Cancer Res. 2008 Dec. 15; 68(24):10024-7.)
On the contrary, little is known about the TET2 protein, which is encoded by a gene located on the 4q24 chromosomal region, and the TET3 protein, which is encoded by a gene located on the 2p12 chromosomal region.
More specifically, the Ten Eleven Translocation oncogene number 2 (TET2) has been designated recently (Lorsbach et al, Leukemia 2003). The TET2 gene located on the chromosomal region 4q24, comprises 11 exons spread over >130 Kb and is normally widely expressed. This gene is referenced with the accession number ID 57790, and its cDNA (Accession number NM_001127208, SEQ ID NO:1) is encoding a protein of 2002 amino acids (Accession number NP_001120680, SEQ ID NO:2).
The TET2 protein shares two highly conserved regions with a single orthologous Drosophila predicted protein. These regions are i) a 310 amino acid region located near the center of the protein TET2 (amino acids 1134 to amino acid 1444), and ii) a second 80 amino acid region located near the carboxyterminal end of the protein TET2 (corresponding to amino acid 1843 until amino acid 1922) (these regions are highlighted in FIG. 1). The predicted sequence of TET2 did not reveal any motif corresponding to an identified function.
Applicants report herein that one or both copies of the Ten Eleven Translocation 2 (TET2) gene are often inactivated/modified by acquired mutations in MPD, MDS and CMML but also in lymphoma. These events target the hematopoietic stem cell and indicate an important function for TET2 as a tumor suppressor gene in myeloid or lymphoid neoplasms.